TEHRAN (Reuters) - A group of Iranian hard-line clerics is signing up volunteers to fight in the Gaza Strip in response to Israel's air strikes that have killed at least 300 Palestinians, a news agency reported on Monday.

"From Monday the Combatant Clergy Society has activated its website www.rohaniatmobarez.com for a week to register volunteers to fight against the Zionist regime (Israel) in either the military, financial or propaganda fields," the semi-official Fars news agency said.

Israel patrols the coastal waters around Gaza and has declared areas around the enclave a "closed military zone."

The hard-line Iranian group, which is headed by some leading clergy, says it has no affiliation with the government and was formed shortly after Iran's 1979 Islamic revolution.

Supreme Leader Ayatollah Ali Khamenei issued a religious decree to Muslims around the world on Sunday, ordering them to defend Palestinians in Gaza against Israeli attacks "in any way possible."

A religious decree is an official statement by a high-ranking religious leader that commands Muslims to carry out its message. While there is no religious and legal force behind it, Khamenei is respected by many Iranian and non-Iranian Shi'ites.

Iran refuses to recognize Israel, which accuses Tehran of supplying Hamas Islamists with weapons. Iran denies the claim, saying it only provides moral support to the group.

Israel said the strikes, that have killed 307 Palestinians, were launched in response to almost daily rocket and mortar fire from the Hamas-ruled Gaza Strip after the Islamist Hamas group ended a six-month ceasefire a week ago.

Fars said the hard-line group provided volunteers with a registration document called "Registration form for dispatching volunteers to Gaza." It said more than 1,100 people so far had registered for military service against Israel.

Khamenei said on Sunday that whoever was killed in the fight to defend Palestinians was "considered a martyr."

Iran will send its first ship carrying aid to the Gaza Strip on Monday, Foreign Ministry spokesman Hassan Qashqavi said.

"Iran has dispatched its first plane load of aid, including medicine, to Gaza on Sunday. The second cargo is on the verge of being dispatched," Qashqavi told reporters on Monday. "The first aircraft arrived in Egypt last night."

Israel, which patrols the coastal waters around Gaza, tightened its blockade of the Gaza Strip two years ago after Hamas won a parliamentary election.

The Jewish state turned back a Libyan ship from delivering humanitarian supplies to Gaza earlier this month.

Tens of thousands of Iranians protested on Monday to condemn the Israeli attacks on the Gaza Strip, which began with air strikes on Saturday.

Protesters burned Israeli and U.S. flags and demanded a stronger response from international organizations to stop Israel's raids, a Reuters witness said.

They also called on Islamic countries to boycott "Zionist companies."